In this question - Why was the Triwizard Cup used as the portkey?, it is answered, (and accepted by everyone who commented ), that the intention of going over so elaborate a procedure to make Harry hold a portkey is to keep Voldemort's return to power a secret. But isn't there a simple way (maybe many more) that would give away his return ? Like the dark mark on Snape's arm? Wouldn't Snape tell Dumbledore right away when he sensed the summon to the graveyard? Mind you, Voldemort at that time thought Snape had "left him forever", so he didn't count on him keeping it a secret from Dumbledore. So, anybody got anything for this?
The Dark Mark spell (Morsmordre) was created by Lord Voldemort and shrouded in mystery. It's unlikely that how it worked for sure was known to anyone but Voldemort and, to a lesser extent, the Death Eaters themselves. It fading and then flaring up again could have meant anything as far as the Ministry was concerned. After all, the Mark hadn't faded completely in 1981, but everyone (but Dumbledore) still assumed Voldemort was quite dead.
I think the main point of why The Death Eaters and Voldemort were trying to keep it secret was partly so that it was only suspected that Voldemort was back to power and also so that no one knew he wasn't just alive again but just as (if not more because he had Harry's blood) powerful as he ever was. What I am saying is that Snape's Dark Mark would have given away the return but not the exact time or the details.